This invntion relates to a process for the separation of a gas containing hydrocarbons.
Propane and heavier hydrocarbons can be recovered from a variety of gases, such as natural gas, refinery gas, and synthetic gas streams obtained from other hydrocarbon materials such as coal, crude oil, naphtha, oil shale, tar sands, and lignite. Natural gas usually has a major proportion of methane and ethane, i.e. methane and ethane together comprise at least 50 mole percent of the gas. The gas also contains relatively lesser amounts of heavier hydrocarbons such as propane, butanes, pentanes, and the like as well as hydrogen, nitrogen, carbon dioxide and other gases.
The present invention is generally concerned with the recovery of propane and heavier hydrocarbons from such gas streams. A typical analysis of a gas stream to be processed in accordance with this invention would be, in approximate mole percent, 86.9% methane, 7.24% ethane and other C.sub.2 components, 3.2% propane and other C.sub.3 components, 0.34% isobutane, 1.12% normal butane, 0.19% iso-pentane, 0.24% normal pentane, 0.12% hexanes plus, with the balance made up of nitrogen and carbon dioxide. Sulfur containing gases are also sometimes present.
The cryogenic expansion process is now the preferred process for the separation of ethane and heavier hydrocarbons from natural gas streams because it provides maximum simplicity, ease of start-up, operating flexibility, good efficiency and good reliability. The cryogenic expansion process is also preferred for the separation of propane and heavier hydrocarbons from natural gas streams while rejecting the ethane into the residue gas stream with the methane. In fact, it is quite common to see the same basic processing scheme used for either ethane recovery or propane recovery, with only the heat exchanger arrangement modified to accommodate the different operating temperatures within the process. U.S. Pat. Nos. 4,278,457, 4,251,249 and 4,617,039 describe relevant processes.
In recent years the fluctuations in both the demand for ethane as a liquid product and in the price of natural gas have created periods in which ethane was more valuable as a constituent of the residue gas streams from gas processing plants. This has resulted in the desire for gas processing facilities to maximize propane and heavier hydrocarbon recovery while, at the same time, maximizing the rejection of ethane into the residue gas stream. Although many variations of the turbo-expander process have been used in the past for propane recovery, they have usually been limited to propane recoveries in the mid-eighty percent to lower ninety percent range without excessive horsepower requirements for residue compression and/or external refrigeration. Although propane recoveries can be improved somewhat by allowing some of the ethane to be recovered in the liquid product, usually a significant percentage of the inlet ethane must leave in the liquid product to provide a small improvement in propane recovery. It is, therefore, desirable to have a process which is capable of recovering propane and heavier components from a gas stream in which only a minor amount of propane is lost to the residue gas while at the same time rejecting essentially all of the ethane.
In a typical cryogenic expansion process, the feed gas under pressure is cooled in one or more heat exchangers by cold streams from other parts of the process and/or by use of external sources of refrigeration such as a propane compression-refrigeration system. The cooled feed is then expanded to a lower pressure and fed to a distillation column which separates the desired product (as a bottom liquid product) from the residue gas which is discharged as column overhead vapor. It is the expansion of the cooled feed which provides the cryogenic temperatures required to achieve the desired product recoveries.
As the feed gas is cooled, liquids may be condensed, depending on the richness of the gas, and these liquids are typically collected in one or more separators. The liquids are then flashed to a lower pressure which results in further cooling and partial vaporization. The expanded liquid stream(s) may then flow directly to the distillation column (deethanizer) or may be used to provide cooling to the feed gas before flowing to the column.
If the feed gas is not totally condensed (usually it is not), the vapor remaining after cooling can be split into two or more parts. One portion of the vapor is passed through a work expansion machine or engine, or expansion valve, to a lower pressure. This results in further cooling of the gas and the formation of additional liquids. This stream then flows to the distillation column at a mid-column feed position.
The other portion of the vapor is cooled to substantial condensation by heat exchange with other process streams, e.g. the cold distillation column overhead. This substantially condensed stream is then expanded through an appropriate expansion device, typically an expansion valve. This results in cooling and partial vaporization of the stream. This stream, usually at a temperature below -120.degree. F., is supplied as a top feed to the column. The vapor portion of this top feed is typically combined with the vapor rising from the column to form the residue gas stream. Alternatively, the cooled and expanded stream may be supplied to a separator to provide vapor and liquid streams. The vapor is combined with the column overhead and the liquid is supplied to the column as a top column feed.
In the ideal operation of such a separation process, the residue gas leaving the process will contain substantially all of the methane and C.sub.2 components found in the feed gas and essentially none of the C.sub.3 components and heavier hydrocarbon components. The bottom product leaving the deethanizer will contain substantially all of the C.sub.3 components and heavier components and essentially no C.sub.2 components and lighter components.
In practice, however, this situation is not obtained due to the fact that the deethanizer is operated basically as a stripping column. The residue gas product consists of the vapors leaving the top fractionation stage of the distillation column together with the vapors not subjected to any rectification. Substantial losses of propane occur because the top liquid feed contains considerable quantities of propane and the heavier components, resulting in corresponding (equilibrium) quantities of propane and heavier components in the vapor leaving the top fractionation stage of the deethanizer. The loss of these desirable components could be significantly reduced if the vapors could be brought into contact with a liquid (reflux), containing very little of the propane and heavier components, which is capable of absorbing propane and heavier hydrocarbons from the vapors. The present invention provides the means for accomplishing this objective and, therefore, significantly improving the recovery of propane.
In accordance with the present invention, it has been found that C.sub.3 recoveries in excess of 99 percent can be maintained while providing essentially complete rejection of C.sub.2 components to the residue gas stream. In addition, the present invention makes posiible essentially 100 percent propane recovery at reduced energy requirements, depending on the amount of ethane which is allowed to leave the process in the liquid product. Although applicable at lower pressures and warmer temperatures, the present invention is particularly advantageous when processing feed gases in the range of 600 to 1000 psia or higher under conditions requiring column overhead temperatures of -85.degree. F. or colder.